1. Field of the Invention
This invention relates to laser shock peening an article and, more particularly, to apparatus and methods for minimizing mist between a laser shock peening laser and the article.
2. Description of Related Art
Laser shock peening or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of an article. Laser shock peening typically uses one or more radiation pulses from high and low power pulsed lasers to produce an intense shock wave at the surface of an article similar to methods disclosed in U.S. Pat. No. 3,850,698 entitled xe2x80x9cAltering Material Propertiesxe2x80x9d; U.S. Pat. No. 4,401,477 entitled xe2x80x9cLaser Shock Processingxe2x80x9d; and U.S. Pat. No. 5,131,957 entitled xe2x80x9cMaterial Propertiesxe2x80x9d. Laser shock peening, as understood in the art and as used herein, means utilizing a pulsed laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface by producing an explosive force at the impingement point of the laser beam by an instantaneous ablation or vaporization of a thin layer of that surface or of a coating (such as tape or paint) on that surface which forms a plasma.
Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following U.S. Pat. No.: 5,756,965 entitled xe2x80x9cOn The Fly Laser Shock Peeningxe2x80x9d; U.S. Pat. No. 5,591,009 entitled xe2x80x9cLaser shock peened gas turbine engine fan blade edgesxe2x80x9d; U.S. Pat. No. 5,531,570 entitled xe2x80x9cDistortion control for laser shock peened gas turbine engine compressor blade edgesxe2x80x9d; U.S. Pat. No. 5,492,447 entitled xe2x80x9cLaser shock peened rotor components for turbomachineryxe2x80x9d; U.S. Pat. No. 5,674,329 entitled xe2x80x9cAdhesive tape covered laser shock peeningxe2x80x9d; and U.S. Pat. No. 5,674,328 entitled xe2x80x9cDry tape covered laser shock peeningxe2x80x9d, all of which are assigned to the present Assignee.
Laser peening has been utilized to create a compressively stressed protective layer at the outer surface of an article which is known to considerably increase the resistance of the article to fatigue failure as disclosed in U.S. Pat. No. 4,937,421 entitled xe2x80x9cLaser Peening System and Methodxe2x80x9d. These methods typically employ a curtain of water flowed over the article or some other method to provide a plasma confining medium. This medium enables the plasma to rapidly achieve shockwave pressures that produce the plastic deformation and associated residual stress patterns that constitute the LSP effect. The curtain of water provides a confining medium, to confine and redirect the process generated shockwaves into the bulk of the material of a component being LSP""D, to create the beneficial compressive residual stresses.
The pressure pulse from the rapidly expanding plasma imparts a traveling shock wave into the component. This compressive shock wave caused by the laser pulse results in deep plastic compressive strains in the component. Laser shock peening is typically performed in a cell or chamber including an enclosure which has walls. Vapors and a mist are produced by the laser shock peening process and fill the chamber. Mist from preceding laser beam shots produce a local mist that reduces the efficiency and power of the beam hitting the laser shock peened surface of the article being laser shock peened. This mist also causes the successive shots to bloom which also interferes with the laser shock peening process thereby reducing the efficacy of each successive laser beam shot. The vapors and mist also cause ionization of the laser beam before it reaches the target area or laser shock peening area on the article or work piece.
High energy laser beams, from about 20 to about 50 joules, or low energy laser beams, from about 3 to about 10 joules, have been used and other levels are contemplated. See, for example, U.S. Pat. No. 5,674,329 (Mannava et al.), issued Oct. 7, 1997 (LSP process using high energy lasers) and U.S. Pat. No. 5,932,120 (Mannava et al.), issued Aug. 3, 1999 (LSP process using low energy lasers). The combination of the energy of the laser and the size of the laser beam provides an energy density or fluence that is usually about 200 J/cm2. Laser shock peened spots are typically formed in overlapping rows of overlapping spots. Typically, overlaps of about 30% of diameters between both spots in a row and between spots in adjacent rows are used. The laser shock peened spots and laser beams are typically circular in shape but may have other shapes such as oval or elliptical (see U.S. Pat. No. 6,541,733, entitled xe2x80x9cLaser Shock Peening Integrally Bladed Rotor Blade Edgesxe2x80x9d by Mannava, et al., issued Apr. 1, 2003.
It is highly desirable to have a laser shock peening apparatus that reduces mist and vapors in a laser shock peening area. It is also desirable to have a laser shock peening apparatus that reduces or prevents ionization of the laser beam before it reaches the target area or laser shock peening area on the article or work piece.
A laser shock peening apparatus includes a laser unit having a laser beam source for generating a laser beam along a laser beam centerline, a beam tube surrounding at least a portion of the beam centerline, and a beam aperture located at an exit of the beam tube. An exemplary embodiment of the apparatus includes a final beam optical lens mounted within the beam tube upstream of the aperture. A gas purging means flows purge gas into the tube between the final beam optical lens and the aperture. One exemplary embodiment of the gas purging means includes a purge gas inlet disposed though the tube between the final beam optical lens and the aperture and a purge gas supply hooked up to the purge gas inlet. Two particularly useful types of the purge gas are air and Nitrogen.
A converging section of the tube is located between the final beam optical lens and the aperture. The tube converges in a downstream direction from the final beam optical lens towards the aperture. The exemplary embodiment of the apparatus further includes at least one telescoping section in the tube between the final beam optical lens and the aperture and, in a more particular embodiment, the telescoping section is in the converging section of the beam tube.
In the exemplary embodiment of the apparatus, the final beam optical lens has a focal number less than 8, the focal number being defined as a ratio of a focal length of the final beam optical lens to a diameter of the lens. The final beam optical lens has a focal point located past the beam aperture outside of the beam tube. In more particular embodiments of the apparatus, the focal number less than 7 or about 5.
The exemplary embodiment of the apparatus further includes a gas knife located between the aperture and the focal point of the lens. The gas knife is used for flowing a large volume of clearing gas across the laser beam between the aperture and the focal point.
The exemplary embodiment of the apparatus is used in conjunction with a laser shock peening cell having an enclosure and a laser shock peening area within the enclosure. The beam tube and the gas knife are disposed within the enclosure while the laser beam source may be located outside of the enclosure. A fluid nozzle is located proximate to and directed towards the laser shock peening area. A drain catch is located within the enclosure under the laser shock peening area and a fluid receptacle, such as a tank, is located outside the enclosure. A vacuum line leads from the drain catch to the fluid receptacle for draining liquid runoff from the fluid nozzle in the open drain catch into the fluid receptacle.
The laser shock peening apparatus reduces mist and vapors in a laser shock peening area and reduces or eliminates ionization of the laser beam before it reaches the target area or laser shock peening area on the article or work piece.